In mobile communication systems, one base station (BS) generally provides services to a plurality of user equipments (UEs). The BS schedules user data on the plurality of UEs to transmit scheduling information to the UEs together with the user data. The scheduling information is control information regarding scheduling. A channel for carrying the control information is referred to as a control channel. A channel for carrying the user data is referred to as a data channel. The BS transmits the control information to each UE through a downlink control channel. Each UE finds control information of each UE by searching for the control channel, and processes data of each UE by using the control information.
The control information may have a variable size. That is, since different control information may be required by each UE, the size of control information may differ in each UE. In addition, even in the same UE, the size of required control information may change depending on time or frequency.
The control information may include scheduling information, information indicating whether data is for unicast, broadcast, or multicast, and multiple input multiple output (MIMO) control information. The scheduling information includes information regarding resource allocation for downlink or uplink transmission and modulation/coding information. The MIMO control information includes not only an acknowledgment (ACK)/negative-acknowledgment (NACK) signal indicating a success/failure of data transmission and a channel quality indicator (CQI) indicating downlink channel quality but also a precoding matrix index (PMI), a rank indicator (RI), etc.
If radio resources are consumed in a large amount to transmit a control signal, there may be a limit in increasing a transfer rate of the user data, and an excessive overhead may be caused in a control channel having limited radio resources. Therefore, it is preferable that the radio resources are required in a small amount when the control signal is transmitted. When the control signal is not properly transmitted, the BS or the UE cannot even know whether related user data is transmitted or not. Therefore, the control signal needs to be transmitted with high reliability. In a situation where limited radio resources are used, it can be seen that an amount of radio resources required for the control signal has a trade-off relation with transmission reliability of the control signal.
That is, for high reliability, it is preferable that a modulation and coding level of a downlink physical control channel is increased and more radio resources are required. However, since the radio resources are limited, the number of UEs that can be processed in one data transmission frame may be limited. On the other hand, a lower modulation and coding level has to be used to process more UEs in one data transmission frame, which may result in decrease in reliability of the control signal in a poor channel condition.
Accordingly, there is a need for a method capable of effectively using radio resources by decreasing an amount of radio resources required for scheduling information and capable of ensuring reliable transmission of control signals.